The effects on immune elimination of helminthic parasites from mice exposed to various chemicals of environmental concern are being studied. Evaluation of adult worm longevity in exposed animals indicated that selected chemicals adversely affect the ability of treated hosts to respond normally to a helminthic infection. The T. spiralis model offers an extremely sensitive indicator for determining altered host susceptibility following chemical exposure and may play a role in increased human parasitic infection in persons exposed inadvertently to selected chemicals.